1. Field of the Invention
The present invention relates to a solid-state imaging device having a color filter, a solid-state imaging apparatus using in combination the solid-state imaging device and an interference-type infrared cut filter, and a digital camera.
2. Description of the Related Art
A digital camera, such as a digital still camera or a digital video camera, is equipped with a solid-state imaging device, such as a CCD or MOS, and converts an optical image entering the camera by way of a lens into an electric signal by means of the solid-state imaging device. A plurality of photoelectric conversion elements (e.g., photodiodes; the elements will hereinafter be described as “pixels”) are formed in an array pattern on the surface of the color solid-state imaging device. Any color filter of an R filter, a G filter, and a B filter is formed on each pixel in the case of a primary-color-based color solid state imaging device.
FIG. 13 is a graph showing spectral sensitivity of the foregoing color solid-state imaging device. A pixel on which the R filter is formed (hereinafter also referred to as an “R pixel”) outputs an electric signal corresponding to the quantity of light of a red component having passed through the R filter. Likewise, a pixel on which the G filter is formed outputs an electric signal corresponding to the quantity of light of a green component of the incident light having passed through the G filter. A pixel on which the B filter is formed outputs an electric signal corresponding to the quantity of light of a blue component of the incident light having passed through the B filter.
However, pigment or dye which can be used for the R filter fails to perform idealistic absorption of infrared radiation. For this reason, a signal output from the R pixel includes a signal corresponding to a large quantity of incoming infrared radiation.
An infrared cut filter has hitherto been inserted in front of the color-solid-state imaging device, thereby blocking the infrared radiation, which cannot be blocked by the R filter.
The infrared cut filter is roughly classified into two types of filters. Namely, one type of filter is a color glass filter, and the other type of filter is an interference-type infrared cut filter utilizing a reflection film. FIG. 14 is a graph showing spectral transmission factors of respective infrared cut filters. A graph of a spectral transmission factor of each glass filter is represented by a characteristic line I, and the spectral transmittance factor of the interference-type infrared cut filter is represented by a characteristic line II.
As can be seen from FIG. 14, the color glass filter absorbs red light in a visible range. The interference-type infrared cut filter has a characteristic of sharply cutting infrared radiation whose wavelength is higher than a cut wavelength (represented by a wavelength at which transmittance becomes 50%).
FIG. 15 is a graph showing spectral sensitivity achieved while the infrared cut filter is placed in front of the color solid-state imaging device. It is understood that the spectral sensitivity—which is indicated by a solid line and uses an interference-type infrared cut filter—shows a sharp decrease in the vicinity of the cut wavelength [this example uses an interference-type infrared cut filter (labeled IR655) manufactured such that the cut wavelength appears at 655 nm] and that little decrease arises in the spectral sensitivity at wavelengths which are shorter than the cut wavelength.
In contrast, dotted lines show spectral sensitivity achieved through use of the color glass filter, and it is understood that a decrease in the sensitivity of a red color (R) is great. Further, a decrease has also arisen in the sensitivity of green color (G). When the sensitivity of R, that of G, and that of B, which are achieved through use of the interference-type infrared cut filter, are presumed to be 100, the sensitivity of R, that of, G, and that of B, which are achieved through use of the color glass filter, particularly show a decrease in red color, as shown in Table 1 provided below.
TABLE 1COMPARISON OF CAMERA SENSITIVITIES (sensitivityachieved through use of IR655 is taken as 100)CCD/IR CUT FILTERREDGREENBLUECCD (FIG. 13)/IR655100100100CCD (FIG. 13)/589198GLASS FILTER
For this reason, the interference-type infrared cut filter involving a small decrease in sensitivity is frequently used as an infrared cut filter to be used in combination with the color solid-state imaging device.
However, the interference-type infrared cut filter has a problem of occurrence of a change in a spectral characteristic of a light ray when the light ray has obliquely entered the interference-type infrared cut filter. Specifically, the problem is that, as incident light enters obliquely, the cut wavelength shifts toward shorter wavelengths. A characteristic line II shown in FIG. 14 shows a case where incident light has entered the surface of a plate-like interference-type infrared cut filter at right angles (i.e., at an incident angle of 0°). A characteristic line II′ shows a characteristic achieved when the incident angle of the incident light assumes an angle of 13°. The cut wavelength of the characteristic line II′ has shifted toward the shorter wavelengths by about 5 nm as compared with the cut wavelength of the characteristic line II.
Miniaturization and slimming down of a recent digital still camera have been pursued, and the digital still camera is built in a portable cellular phone. For reasons of such progress in miniaturization of the digital still camera, a short focus lens is used in front of a color solid-state imaging device equipped with the interference-type infrared cut filter. Therefore, the incident light having passed through the lens assumes an incident angle of 0° (i.e., enters perpendicularly) at the center of the solid-state imaging device. However, the incident light enters obliquely in the periphery of the solid-state imaging device. A greatly-miniaturized digital camera assumes an incident angle of about 13°. However, when miniaturization of the digital camera is pursued, the incident angle becomes 13° or more.
As can be seen from the characteristic line II′ shown in FIG. 14, the shift in cut wavelength, which is induced as a result of oblique entrance of incident light into the interference-type infrared cut filter, does not extend to a green range or a blue range. Therefore, a decrease in the sensitivity of green color or a decrease in the sensitivity of blue color is not induced. However, a shift in cut wavelength causes a decrease in the sensitivity of red color. Specifically, a decrease arises in the sensitivity of red color with increasing proximity to the periphery of the solid-state imaging device, thereby causing inconsistencies in color (color shading). For instance, when an image which is totally gray is photographed, a deficiency in red arises at the edge of the image, and hence the edge of the image assumes a cyan-toned color.
Therefore, according to the related-art technique described in, e.g., JP-A-11-352324, the interference-type infrared cut filter to be disposed in front of the solid-state imaging device is formed not in a plate-like shape but a curved shape, thereby rendering smaller the incident angle of incoming light at the periphery of the solid-state imaging device.
The interference-type infrared cut filter is formed into a curved shape, thereby preventing the incident angle of light entering the solid-state imaging device from becoming greater around the periphery of the solid-state imaging device. As a result, color fading, which is responsible for a shift in cut wavelength of the interference-type infrared cut filter, can be diminished.
However, when the interference-type infrared cut filter is formed into a curved shape, the thickness of the interference-type infrared cut filter is increased, which in turn hinders miniaturization and slimming down of the digital camera and adds to costs incurred in manufacturing the interference-type infrared cut filter.